BSTJ 31: 5. September 1952: Nationwide Numbering Plan. (Nunn, W.H.)

Nationwide Numbering Plan

By W. 11. NUNN

(Manuscript received May 15, 1952)

In telephone language a numbering plan gives each telephone in a city,
a town, or a geographical area an identity or designation different from
that given any other telephone in the same area. There is a wide variation
in the types of numbering arrangements in use today in the Bell System,
and this paper gives the reasons for this diversity, and examples of the
various numbering plans now in use. With the introduction of modern toll
switching facilities and the extension of toll dialing to nationwide scope,
it was realized that an improvement in the method of dialing toll calls to
distant cities was essential in order to realize the maximum speed and
accuracy inherent in toll dialing. A nationwide numbering plan covering the
United. States and Canada has been designed. Each of the more than 20,000
central offices in the two countries are to be given a distinctive designation
which identifies that particular office. This designation is to consist of a
regional or area code and a central office code The new switching equipment
for the key points in the toll network is being designed so that any toll opera-
tor, wherever located, will use the sa?ne designation or code for reaching a
given office. The combination involved in laying out these areas and the
composition of the area codes are presented. A total of 152 codes are available
of which approximately 90 are assigned to the present numbering plan areas.
Ultimately each central office will be given a type of number consisting
of an office name and five numerical digits, such as LOcust Jj-5678, in
which the first two letters of the office name become the two letters of the
central office code. The entire program will take a considerable number of
years to realize, bid is one which must be accomplished in order to achieve
the best results in operator toll dialing and the ultimate goal of nationwide
customer toll dialing.

In telephone language a numbering plan is exactly what the name im-
plies, a plan or system of giving each telephone in a city, a town or any
geographical area an identity or designation which is different from that
given every other telephone in this same area. This designation is the

851

852 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952

telephone number; it appears in the directory and in most cities on the
telephone instrument itself. It is the address of the telephone in the
telephone network. Just as it is essential for efficient postal and delivery
service to have streets and house numbers clearly marked, it is important
for good telephone service that the telephone numbering plan be such
that it will be used with convenience and accuracy by the telephone
customer.

A telephone number is comprised of two elements, a designation for
the central office to which the telephone is connected and a number
within the central office which identifies one particular telephone from
all others served by that office. If there is only one central office in the
city or town, the office designation is frequently omitted. A dial office is
designed to serve up to 10,000 numbers with a limitation of four digits.
Typical numbers are therefore MAin 2-1234, ADams-2345, 5-6789 and
3456, the office designations being MAin 2, ADams and 5 with the last
four digits in all cases representing the number within the central office.

There is a wide variation in the types of numbering arrangements
in use today in the Bell System. This diversity arises from the fact that
telephone communities vary greatly with respect to the number of
telephones served, ranging all the way from New York City with its
more than three million telephones and three hundred central offices
to small villages and rural communities with perhaps a few score or a
few hundred telephones.

In the 1920's when the Bell System embarked upon its program of
converting local offices to dial operation each exchange or city was in
general an entity unto itself. Customers dialed local calls within their
own city but all calls involving a toll or multi-unit charge required
handling by operators for timing and ticketing. There was no advantage,
therefore, in making a numbering plan for a given city more compre-
hensive than required to serve the telephones and central offices in that
city with a suitable allowance for the expected growth. Thus there were
formed a multitude of local dial communities, large and small, within
which customers could dial their own calls and connections between these
telephone communities were established by operators.

Over the years these basic numbering plans which were originally
established for local dialing have in many of the cities proved inadequate
to furnish as many office codes as later events have shown are required.
This is due to a variety of causes. The station growth in many places has
outstripped all expectations and the number of central offices required
to serve this unprecedented demand for service consume many more
office codes than the original plans provided for.

NATIONWIDE NUMBERING PLAN 853

In many places local service areas were changed so that customers
could call into contiguous exchanges at local rates. To enable customers
to dial into these neary-by places the original numbering plans required
expansion to include this increased number of offices. In addition, with
the advance in the telephone art many cities introduced equipment for
automatic charging on multi-unit and short haul toll calls so that cus-
tomers could dial such calls directly instead of placing them with an
operator for completion. In order to enable customers to dial these calls,
it was necessary to expand the original city numbering plans to encom-
pass wider and wider geographical areas.

In expanding the various types of numbering plans to serve a larger
number of central offices than were originally anticipated, various ex-
pedients were resorted to. In the largest cities having three-letter office
codes a numeral was substituted for the third letter thus very materially
increasing the code capacity from about 325 to about 500 and making it
possible to form a number of codes using the same office name. The name
CANal for example, instead of serving but one office may serve a number
of offices, CAnal 2, CAnal 3, CAnal 4, etc. In the medium size cities
having two-letter codes, expansion meant adding a digit to the code to
all or in some cases to only a part of the offices in the city.

The five-digit places were usually expanded by adding a digit to
some of the numbers so that some of the telephones had five digits and
others six digits in their numbers.

As a result of choosing originally a numbering plan which at the time
seemed adequate and most suitable for the cit3 r involved and in many
cases being forced to expand to meet changing needs, we now have in the
Bell System a considerable variety of different numbering plans. These
are given in Table I. The numbering plans given are all adequate to
serve the present local dialing needs for the cities in which they appear.

Having reviewed the numbering plan situation as it exists today in
the various cities and towns, let us turn to the problem of handling toll
calls. Under ringdown operation there is an operator at the outward toll
center where the call originates and another operator at the terminating
or inward toll center. On built-up toll connections there are additional
operators at each intermediate toll switching point. The inward toll
operators, who are familiar with the numbering plans in the offices served
by their particular toll center, can be relied upon to connect to the de-
sired station even though there is uncertainty on the part of the calling
customer or the outward toll operator regarding the precise pronuncia-
tion or spelling of the name of the called office or the particular form of
numbering system used at the called city.

854 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952

Under operator toll dialing the inward operator is replaced by dial
switching equipment under the control of the outward operator; hence
the outward operator has no one to rely upon but herself in completing a
toll connection to a distant city. With the present method the operator
dials a code for each circuit group in the connection followed by the
number of the called party which may consist of any number of digits
from three to seven. The operator must refer to her position bulletin or
to a routing operator for the correct circuit group codes unless she hap-
pens to remember them. Where the office to be reached has central office
names, the operator must rely on routing information to determine how
many letters of the name are to be dialed. The great variation in the
number of digits to be dialed on different calls is a source of some dif-
ficulty and confusion to the operators.

The present system of operator toll dialing by which operators use
codes depending upon the routes to reach a desired destination, is a
great improvement over the old manual handling methods. However,
with the introduction of more modern toll switching facilities and the
nationwide extension of toll dialing, it was realized that an improvement
in the methods for dialing toll calls to distant cities was essential in order
to realize the maximum speed and accuracy inherent in toll dialing.

These handicaps in the present toll dialing methods are to be overcome
by establishing a nationwide numbering plan covering the United States
and Canada by which each of the more than 20,000 central offices in the
two countries is to be given a distinctive designation which identifies
that particular office and that office only. This designation is to consist of

Table I — Different Types of Numbering Plans

Place

Philadelphia, Pa.
Los Angeles, Cal.

Indianapolis, Ind.
El Paso, Texas

San Diego, Cal.

Des Moines, Iowa

Binghamton, N. Y.
Manchester, Conn.

Winchester, Va.
Ayer, Mass.

Jamesport, N. Y.

Directory Listing

LOcust 4-5678
PArkway 2345 and

REpublic 2-3456
MArket 6789
PRospect 2-3456

and 5-5678
Franklin 9-2345
Franklin 6789
4-1234 and 62-2345

2-5678

5678 and 2-2345

3456

629 and 2345

325

Customer Dials Ordinarily Referred to as

LO 4-5678
PA 2345 and

RE 2-3456
MA 6789
PR 2-3456 and

5-5678
F 9-2345
F6789
4-1234 and 62-

2345
2-5678
5678 and 2-2345

3456

629 and 2345

325

Two-five
Combined two-four

and two-five
Two-four
Combined two-five

and five digit
One letter, four and

five digit
Combined five and

six digit
Five digit
Combined four and

five digit
Four digit
Combined three and

four digit
Three digit

NATIONWIDE NUMBERING PLAN 855

two elements, a regional or area code and a central office code. Any
outward toll operator, wherever located, will use that same designation
in reaching that office through the dial toll switching network.

In a sense, all of the thousands of offices involved are to be treated
as though they were contained in one huge multi-office city. Toll opera-
tors will use the area code and the office code in reaching an office situated
outside her own numbering plan area, while on calls to points within
her own numbering plan area she will dial only the number as listed for
toll in the directory. In principle the method employed is to divide the
two countries geographically into numbering plan areas and to give
each of these areas a distinctive code. Refer to Fig. 1. Within each of
these numbering plan areas each office will have a code unlike that of any
other office in the same numbering plan area and also unlike any area
code. Hence for toll dialing purposes each office will have an area code
and central office code which will form a combination unlike that of any
other central office in the two countries.

In this geographical division into numbering plan areas, border lines
between states and between Canadian provinces have generally been used
as numbering area boundaries. Since about 500 central offices are the
maximum number which can be served in a numbering plan area, it is
necessary to divide the larger and more populous states and provinces
into two or more areas making, of course, due allowance for growth.
New York state with the largest number of central offices is divided
into six numbering plan areas; Pennsylvania, Illinois, Texas and Cali-
fornia have four areas each. Other divided states have three or two areas
depending upon the number of offices to be served. Approximately 90
areas are being provided, with 14 states and two provinces served by
two or more numbering plan areas, the remaining states and provinces
by one area each.

In fixing the intrastate numbering plan area boundaries of subdivided
states, among other considerations effort was made to avoid cutting
across heavy toll traffic routes in order to have as much of the toll traffic
as possible terminating in the area in which it originated. The advantage
of arranging the numbering plan areas in this manner is readily apparent
since on this traffic which does not pass an area boundary the area code
is not required.

Let us now consider the composition of the area codes. As indicated
previously they must be of a type which will enable the switching equip-
ment to distinguish them from the codes of central offices.

On the telephone dial plate letters are assigned only to the dial posi-
tions 2 to 9, inclusive (on some dial plates a Z appears on the position

856 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952

but the Z is never used in a central office code), hence any office code
will always avoid a 1 or a in the first two places. The digits 1 and
can therefore be used in area codes to distinguish these from office codes.
It is not practical to use them as initial digits of area codes since custo-
mers dial to reach operators and the local dial equipment is arranged to
ignore an initial 1 for technical reasons. A 1 or in the second place,
however, can be employed in an area code without conflicting with any
central office codes or interfering with any existing practices. Accord-
ingly the area codes will consist of three digits with either a 1 or a as
the middle digit, 516, 201, etc. A few codes of this type are now in use,
leaving a practical total of 152 of these area codes available as compared
to approximately 90 assigned to our present numbering plan areas. This
will provide a comfortable spare for additional future numbering plan
areas or possibly for reaching overseas points which may later be in-
corporated into the toll dialing network.

As shown in Fig. 1, states and provinces such as Montana or Alberta
which are contained in a single numbering plan area will have area
codes with a as the middle digit to distinguish them from areas in
divided states such as Texas where the middle digit will be a 1. This is
to enable toll operators to differentiate between the two classifications
of areas. On calls to single area states the operators will always know that
every call to the state in question uses the one area code, whereas on
calls to subdivided states additional information will be required to de-
termine which of the several area codes should be employed to reach the
particular destination. It is proposed to show on the operator position
bulletin the codes of all single area states and the codes of all frequently
called cities in multi-area states. The area codes of the less frequently
called places in the multi-area will be obtained from a routing operator.

Within each numbering plan area each of the 500 or fewer offices
are to be given a three-digit office code which will be different from that
of any other office code in that same area. Ultimately each central office
will be given a 2-5 type of number consisting of an office name and five
numerical digits, such as LOcust 4-5678, illustrated for Philadelphia.
In the larger cities customers will dial seven digits, LO 4-5678, on local
calls to numbers in the same exchange. In many of the smaller places
the customers on local calls will dial only the numerical digits, the office
name being employed for toll dialing purposes only.

Considering the thousands of central offices which now have numbers
other than the 2-5 type and the fact that to change existing numbering
systems is a difficult and often costly procedure, it will be a number of
years before this ultimate objective is realized. As a practical measure,

NATIONWIDE NUMBERING PLAN 857

therefore, it will be necessary during this interim period, before the
central office names with the 2-5 type of number are established every-
where, to employ for operator toll dialing office codes which in many
cases may not be derived from the customers' telephone number.

In dialing to a combined 2-4 and 2-5 city, for example Los Angeles,
the three-digit office code for the Parkway office which has six digits in
the local number, will be PAR, whereas to reach the Republic 2 office
having seven digits in the local number, the office code will be RE2.
To call a telephone in Winchester, Va., with only four digits in the local
number, the operator will use a code consisting of numerical digits only,
such as 294 which, of course, must be different from every other office
code in this numbering plan area. To secure the particular office code
to be used in reaching an office where the called number does not furnish
complete information, the toll operator must refer to a position bulletin
or the route operator. This reference work, of course, takes time and
therefore imposes a delay in completing the call.

In addition to giving a distinctive three-digit code to each office
within each numbering plan area, each toll center will also be given
a three-digit code to enable outward operators to reach inward informa-
tion, and delayed call operators at toll centers in distant cities. Calls to
these operators will be routed in the same manner as calls to customers
except that the operator codes will be used instead of a station number
and a toll center code in place of a central office code.

The central office names now in use in the various cities in the System
were chosen, generally speaking, on the basis of their suitability for
customer dialing within the city itself. Many of these names are un-
familiar words to operators and customers in distant cities and the use
of these names contributes materially to the operator dialing errors.
This situation is gradually being corrected by using for new offices,
names from a System approved list and replacing existing names which
experience has shown to be particularly troublesome by names from this
list.

While numbering plans are important in operator toll dialing, they
play an even more essential part in the dialing of toll calls by customers.
Operators can be trained to adapt their dialing procedures to the type
of local numbering system encountered in the called city even though
more time is consumed and more errors result than would be the case
if all telephone numbers were of a uniform type. Customers, however,
could not be expected to follow any plan which requires a variety of
different procedures to be used in reaching different cities. Only a num-
bering system which is readily understandable and which customers find

858 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952

convenient to use and one which they can use with a very high degree
of accuracy will suffice. The need for accuracy is readily apparent since
with the customer's telephone being given access to the intertoll network
without the intervention of an operator, a call which is misdialed can be
routed to a telephone thousands of miles from the desired destination.

At present customer dialing of toll and multi-unit calls is for the most
part confined to situations where the call can be completed by the use
of the number as listed in the directory without any additional digits
being dialed. In a few cases as from Camden, N. J. to Philadelphia and
certain offices in Northern New Jersey to New York City, the code 11
is prefixed to the listed number. In the case of the current trial of cus-
tomer toll dialing at Englewood, N. J., the customers are using area
codes such as 415 for Oakland, California, 312 for Chicago, etc., dialing
only into those cities which now have the 2-5 type of numbering.

From the Englewood experience it can be confidently predicted that
this form of dialing, i.e., an area code followed by a telephone number
consisting of a uniform number of digits, is one that customers will use
with a reasonable degree of convenience and accuracy. The problem
therefore to meet the requirements for nationwide customer toll dialing,
is to establish universally for all central offices regardless of size and loca-
tion a uniform pattern of numbering for toll purposes. The only form of
number completely filling the needs is the 2-5 system, which is that used
in the largest cities today.

Accordingly, in order to implement the program for customer dialing
of toll calls on a nationwide basis, it will be necessary to place all tele-
phone numbers on a 2-5 basis with the code of each office different from
that of every other office in the same numbering plan area. Thus each of
the 50,000,000 telephones in the United States and Canada will have, for
toll dialing purposes, a distinct identity consisting of ten digits; a three-
digit area code, an office code of two letters of an office name and a
numeral, and four digits of the station number within the office. Typical
numbers for toll dialing would therefore be 601-CA3-4567 or 317-MA7-
6789. As with operator toll dialing, on a toll call which terminates in the
same numbering plan area in which it originates, the area code will be
omitted and the office code and station number — a total of seven digits

will be used.

With this universal 2-5 type of number, local calls in and about the
larger and medium sized exchanges will be completed by dialing the
entire seven-digit number. For many of the smaller places in the more
isolated sections, 5-digit or 4-digit dialing will frequently be employed
where this number of digits will be adequate for all of the telephones

o

-2
-

—
—

\»z

60

NATIONWIDE NUMBERING PLAN 859

in the customers' local dialing area. For these offices with five or four-
digit local dialing and for offices in the larger places served by certain
types of dial equipment, as they are arranged today, it will be necessary
to prefix the dialing of toll calls by a transfer or directing code to permit
the customer getting from the local office into the toll network.

Independent of the advantages of a universal 2-5 numbering plan
for nationwide operator and customer toll dialing, the Bell System has
made considerable progress in this direction over the past several years.
New York and Northern New Jersey adopted 2-5 numbering in 1930
in order to take advantage of the flexibility of office code assignments
and the large code capacity which this type of local numbering provides.
Since World War II many cities and their environs such as Chicago,
Boston, Philadelphia, San Francisco, Oakland, Pittsburgh, Milwaukee,
Providence and a number of smaller cities have followed suit. Presently
about 12 million telephones are in areas which have 2-5 numbering
exclusively in addition to perhaps two million telephones with 2-5
numbers in mixed 2-4 and 2-5 areas. Another five million telephones
are already planned for conversion to 2-5 numbers within the next
several years.

The entire program will take many years to realize but it is one which
must be accomplished in order to achieve the best results in operator
toll dialing and make it possible for a customer at any telephone in the
United States and Canada to reach a telephone anywhere in the two
countries by dialing without the assistance of an operator.